After cleaning and drying of one or more kinds of substrates as a material, the material substrates are joined, in manufacture of a bonded substrate. Then, they are subjected to heat treatment for increasing a joining strength at a joining interface of the joined substrate, thereby they are bonded firmly, and then one of the substrates is made thin. Thereby, a bonded substrate is manufactured. At this time, the drying process conducted after the process of cleaning material substrates is usually carried out by a water displacing method represented by a method comprising putting a substrate into a heated vapor of IPA (isopropyl alcohol) which is one of highly volatile organic solvents, and drying it by increasing a temperature of the substrate to a vapor temperature with replacing moisture adhered on the substrate surface with condensed IPA (hereafter referred to as IPA vapor drying method).
If particles or an organic substance exist on the joining surface of a substrate as a material when joining the material substrates in manufacture of a bonded substrate, the particles and the organic substance will prevent the joining surfaces from being joined, and will form a non-joined part in the joining interface. It causes a void failure and a blister failure in the bonding interface of the bonded substrate after heat treatment for bonding. Especially, when a joining strength at a joining interface is weak, a void failure and a blister failure tend to be caused easily even if the heat treatment for bonding is performed, and a size thereof also tends to become large.
FIG. 2 is a schematic view of a longitudinal section of a SOI wafer as an example of a bonded substrate in which the above failures are generated.
The SOI wafer is formed by stacking an oxide film 10 and an active silicon layer 9 on a base wafer 2, and a non-bonded part is generated in these layers due to a void failure 13 and a blister failure 12.
In a drying process performed after a process of cleaning a substrate as a material in manufacture of a bonded substrate which is conventionally performed, a water displacing method such as the IPA vapor drying method described above is adopted. When a bonded substrate is produced using the substrates dried by the water displacing method, the above-mentioned void failure and blister failure tend to be generated easily, and reduce productivity and yield of the bonded substrate.
The water displacing method is a method of replacing moisture adhered on a substrate surface with a highly volatile organic solvent to dry the substrate quickly. As a solvent for displacement, IPA which is the easiest to use industrially has been used widely.
Furthermore, since the fire might occur when performing the IPA vapor drying method, much plant investment for safety has been required therefor. Moreover, since IPA is a high-grade chemical, it is expensive. Therefore, the manufacture cost gets high when using it.